Structural foamed materials are known, and can be produced by injecting a physical blowing agent into a molten polymeric stream, dispersing the blowing agent in the polymer to form a two-phase mixture of blowing agent cells in polymer, injecting the mixture into a mold having a desired shape, and allowing the mixture to solidify therein. A pressure drop in the mixture can cause the cells in the polymer to grow. As an alternative to a physical blowing agent, a chemical blowing agent can be used which undergoes a chemical reaction in the polymer material causing formation of a gas. Chemical blowing agents generally are low molecular weight organic compounds that decompose at a critical temperature and release a gas such as nitrogen, carbon dioxide, or carbon monoxide. Under some conditions the cells can be made to remain isolated, and a closed-cell foamed material results. Under other, typically more violent foaming conditions, the cells rupture or become interconnected and an open-cell material results. A sample of standard injection molding techniques described in the patent literature follow.
U.S. Pat. No. 3,436,446 (Angell) describes a method and apparatus for molding foamed plastic articles with a solid skin by controlling the pressure and temperature of the mold.
U.S. Pat. No. 4,479,914 (Baumrucker) describes a foamed article forming method in which a mold cavity is pressurized with gas to prevent premature diffusion of blowing gas from the material injected into the cavity. Pre-pressurization gas is vented during injection of material to be foamed, finally to a vacuum chamber creating a vacuum that draws the material throughout the mold cavity.
Particularly low-density (high void volume) molded polymeric foam materials include expanded polypropylene (EPP) and expanded polystyrene (EPS). Precursors to EPP or EPS can be provided as beads of already partially or fully-foamed polypropylene or polystyrene, respectively. These beads typically are injected into a steam chest mold and fused together to form a structural foam product from the beads. In some cases, further expansion and foaming of the beads occurs within the steam chest (typically with EPS). While EPS and EPP are useful products for many applications, they can have a less than ideal appearance. Lines of fusion between beads can easily be observed by the naked eye in products such as styrene foam cups and other EPP or EPS products.
Microcellular material typically is defined by polymeric foam of very small cell size and various microcellular material is described in U.S. Pat. Nos. 5,158,986 and 4,473,665. These patents describe subjecting a single-phase solution of polymeric material and physical blowing agent to thermodynamic instability required to create sites of nucleation of very high density, followed by controlled cell growth to produce microcellular material. U.S. Pat. No. 4,473,665 (Martini-Vvedensky) describes a molding system and method for producing microcellular parts. Polymeric pellets are pre-pressurized with a gaseous blowing agent and melted in a conventional extruder to form a solution of blowing agent and molten polymer, which then is extruded into a pressurized mold cavity. The pressure in the mold is maintained above the solubility pressure of the gaseous blowing agent at melt temperatures for given initial saturation. When the molded part temperature drops to the appropriate critical nucleation temperature, the pressure on the mold is dropped, typically to ambient, and the part is allowed to foam.
U.S. Pat. No. 5,158,986 (Cha et al.) describes an alternative molding system and method for producing microcellular parts. Polymeric pellets are introduced into a conventional extruder and melted. A blowing agent of carbon dioxide in its supercritical state is established in the extrusion barrel and mixed to form a homogenous solution of blowing agent and polymeric material. A portion of the extrusion barrel is heated so that as the mixture flows through the barrel, a thermodynamic instability is created, thereby creating sites of nucleation in the molten polymeric material. The nucleated material is extruded into a pressurized mold cavity. Pressure within the mold is maintained by counter pressure of air. Cell growth occurs inside the mold cavity when the mold cavity is expanded and the pressure therein is reduced rapidly; expansion of the mold provides a molded and foamed article having small cell sizes and high cell densities. Nucleation and cell growth occur separately according to the technique; thermally-induced nucleation takes place in the barrel of the extruder, and cell growth takes place in the mold.
While the above and other reports represent several techniques associated with the manufacture of microcellular material and the manufacture of material via injection molding, a need exists in the art for improved microcellular injection molding processes.
It is, therefore, an object of the invention to provide injection molding systems and methods effective in producing microcellular structural foams and, in particular, very thin articles. It is another object to provide systems and methods useful in injection molding of microcellular structural foams, but also useful in injection molding of conventional foams and continuous extrusion of microcellular or conventional foams.